The invention relates to a negative working light-sensitive mixture which comprises a diazonium salt condensation product, a polymerizable compound, a photoinitiator and a polymeric binder, and to a recording material prepared therewith which is preferably suitable for preparing planographic printing plates.
Combinations of photopolymerizable mixtures with negative working diazo compounds, in particular diazonium salt polycondensation products, are known from DE-A-2,361,931, DE-A-2,903,270 and DE-A-3,007,212 (=GB-A-2,044,788). Therein either the diazo compound itself acts as a photoinitiator for the polymerization of the monomer, or an additional photoinitiator is added. The advantage is said to be in particular the improved resolution of the copy compared with pure photopolymerizable layers and the increased print run compared with layers which contain only diazo compounds as light-sensitive substances. Moreover, in certain cases it is possible to carry out the development with aqueous solutions or even with pure water.
A similar mixture which contains a binder having cross-linkable allyl side groups is described in EP-A-104,863. The advantage of the mixtures described therein is said to be in particular that the light sensitivity of the recording materials prepared therewith has a reduced temperature dependence. A further advantage is said to be the reduced dependence of the light sensitivity on the time gap between exposure and development. Offset printing plates which have been prepared from these light-sensitive mixtures, however, have the disadvantage of a relatively low run stability and an undesirably high dot gain during printing.
EP-A-167,963 describes a light-sensitive mixture of the above-mentioned constituents which contains as diazonium salt condensation product a polycondensation product units A-N.sub.2 X and B which are linked by intermediate members, preferably methylene groups, A being the radical of an aromatic diazonium compound which is condensable with formaldehyde and B being the radical of a compound which is likewise condensable with formaldehyde but which is free of diazonium groups, in particular the radical of an aromatic amine, of a phenol, or phenol ether, of an aromatic thioether, of an aromatic hydrocarbon, of an aromatic heterocycle compound or of an organic acid amide. The mixture produces long-run printing plates if it contains a water-insoluble binder. In this case, it requires the addition of organic solvents to the developer.
EP-A-211,391 describes a similar mixture which contains as binder the reaction product of a di- or tricarboxylic anhydride with a polymeric organic compound which contains free hydroxyl groups. The printing plates obtained therewith are developable in aqueous solutions without a major concentration of organic solvents and likewise produce long runs. The light-sensitive layers formed from this mixture, however, are tacky at elevated temperature and therefore must be provided with a non-tacky cover layer, for example a cover layer of polyvinyl alcohol. This layer, which acts as a barrier layer to atmospheric oxygen, likewise improves the curing of the light-sensitive layer. However, the cover layer does impair the developability, so that development takes distinctly longer. This is particularly noticeable following prolonged storage or storage at elevated temperature.